Question

An OpenStreetMap xml document is composed (among other things) of a set of “node” elements and a set of “way” elements.

The “node” elements can (optionally) nest “tag” elements.

The “way” elements are composed by an ordered list of “node” elements, referenced by the nested elements “nd”, with their attribute “ref” pointing to the attribute “id” at the “node” elements.

Here an example:

<?xml version="1.0" encoding="UTF-8"?>
<osm version="0.6" generator="CGImap 0.0.2">
  <node id="1726631203" lat="50.8500083" lon="4.3553223" visible="true" version="6" changeset="9938190" timestamp="2011-11-24T22:05:32Z"/>
  ...
  <way id="160611697" user="toSc" uid="246723" visible="true" version="1" changeset="11385198" timestamp="2012-04-22T14:57:19Z">
    <nd ref="1726631203"/>
    <nd ref="1726631223"/>
    <nd ref="1726631213"/>
    <nd ref="1726631205"/>
    <nd ref="1726631185"/>
    <nd ref="1726631203"/>
  </way>
  ...
</osm>

My question is how, using XSLT, could I do the following transformation ?

• Filtering all the node elements that are not referenced by any way element.
• Filtering the way elements that reference node elements not included in the source xml document.
• Changing the attribute “visible” to “false”, to any “node” element not having “tag” children elements.

Any other elements should remain in the generated xml.

Answer 1

My question is how, using XSLT, could I do the following
transformation ?

• Filtering all the node elements that are not referenced by any way element.

• Filtering the way elements that reference node elements not included in the source xml document.

• Changing the attribute “visible” to “false”, to any “node” element not having “tag” children elements.

This transformation fullfills all three requirements:

<xsl:stylesheet version="1.0"
 xmlns:xsl="http://www.w3.org/1999/XSL/Transform">
 <xsl:output omit-xml-declaration="yes" indent="yes"/>
 <xsl:strip-space elements="*"/>

 <xsl:key name="kND-By-Ref" match="way/nd" use="@ref"/>
 <xsl:key name="kNodeById" match="node" use="@id"/>

 <xsl:template match="node()|@*">
     <xsl:copy>
       <xsl:apply-templates select="node()|@*"/>
     </xsl:copy>
 </xsl:template>

 <xsl:template match="node[not(key('kND-By-Ref', @id))]"/>
 <xsl:template match="way[nd[not(key('kNodeById', @ref))]]"/>

 <xsl:template match="node[not(tag)]/@visible">
  <xsl:attribute name="visible">false</xsl:attribute>
 </xsl:template>
</xsl:stylesheet>

when applied on this XML document (suitably created to contain a case for each requirement):

<osm version="0.6" generator="CGImap 0.0.2">
    <node id="1726631203" lat="50.8500083" lon="4.3553223" visible="true"
      version="6" changeset="9938190" timestamp="2011-11-24T22:05:32Z">
      <tag/>
 </node>
    <node id="1726631223" lat="50.8500083" lon="4.3553223" visible="true"
      version="6" changeset="9938190" timestamp="2011-11-24T22:05:32Z"/>
    <node id="ZZZZZZZ" lat="50.8500083" lon="4.3553223" visible="true"
      version="6" changeset="9938190" timestamp="2011-11-24T22:05:32Z"/>
    <way id="160611697" user="toSc" uid="246723" visible="true"
      version="1" changeset="11385198" timestamp="2012-04-22T14:57:19Z">
        <nd ref="1726631203"/>
        <nd ref="1726631223"/>
    </way>
    <way id="160611698" user="toSc" uid="246723" visible="true"
      version="1" changeset="11385198" timestamp="2012-04-22T14:57:19Z">
        <nd ref="1726631203"/>
        <nd ref="1726631223"/>
        <nd ref="1726631213"/>
        <nd ref="1726631205"/>
        <nd ref="1726631185"/>
        <nd ref="1726631203"/>
    </way>
</osm>

the wanted, correct result (all filterings are performed and the visible attribute of one of the node elements is turned to false) is produced:

<osm version="0.6" generator="CGImap 0.0.2">
   <node id="1726631203" lat="50.8500083" lon="4.3553223"
    visible="true" version="6" changeset="9938190" timestamp="2011-11-24T22:05:32Z">
      <tag/>
   </node>
   <node id="1726631223" lat="50.8500083" lon="4.3553223"
    visible="false" version="6" changeset="9938190" timestamp="2011-11-24T22:05:32Z"/>
   <way id="160611697" user="toSc" uid="246723" visible="true"
    version="1" changeset="11385198" timestamp="2012-04-22T14:57:19Z">
      <nd ref="1726631203"/>
      <nd ref="1726631223"/>
   </way>
</osm>

Explanation:

1. The identity rule is overriden by three templates, each implementing one of the three requirements.

2. The two overriding templates that have empty bodies implement the two filtering requirements.

3. We are using keys to conveniently and efficiently find node s by their id attribute and nd s by their ref attribute.

4. The attribute value replacement requirement is implemented in the third overriding template.